Advances in OLED technology have in recent times resulted in an effort to manufacture and optimise display devices based on OLED technology. Despite recent progress in OLED technology, there is still a desire to improve the performance of OLED display devices, and the ease and cost of manufacturing such devices. Improving OLED technology will improve the competitiveness of OLED displays compared with inexpensive liquid crystal displays.
An exemplary OLED display panel having an active matrix structure comprises a matrix of pixel circuits. Each pixel circuit has an OLED device and backplane having matrix wiring and thin film transistors (TFTs) for switching and driving the OLED device.
Such a pixel circuit has difficulty delivering and controlling electric current passed through each OLED device. This is because each drive TFT is comprised of amorphous silicon (a-Si) which has low carrier mobility. The low carrier mobility prevents a small a-Si TFT from delivering a large drive current. Additionally, such an a-Si TFT suffers from great changes in characteristics when subjected to a continuous current application, making control of the drive current difficult.
Discussed below are several approaches which have been proposed to address these difficulties.
A first proposed solution is to use a polycrystalline silicon TFT for the drive TFT. This provides the drive TFT with higher carrier mobility. However, this technology necessitates crystallization annealing processes in order to obtain polycrystalline silicon, generally on a glass substrate. The crystallization annealing is typically conducted through laser heating to temperatures above 1000° C., and it suffers from the disadvantage of a lower throughput as well as lower yield, resulting in a significantly higher cost as compared with a-Si TFT.
A second proposed solution is to use amorphous oxide TFTs for the drive TFTs. This technology uses a non-crystalline semiconductor of metal oxide, such as an oxide of In or Zn, and accordingly has an advantage of eliminating the necessity for crystallization processes such as a laser annealing process. However, this technology also suffers from the disadvantage of significantly lower yield. The low yield is due to a need for approximately 5-micron rule micromachining. There is also a difficulty in using existing a-Si manufacturing lines to manufacture amorphous oxide TFTs.
A third proposed solution is to provide a luminescence transistor by giving the TFT a luminescence function, thereby reducing the number of transistors required in the pixel circuit. An example of such an attempt is disclosed in JP2002-246639 where the TFT has a luminescence signal visible as a narrow line on a surface. However, such a TFT suffers from having a narrow emission area. Narrow line emission has limited scope in applications involving display units. Further, it is difficult to separate the luminescence transistor manufacturing process and the backplane manufacturing process.
A fourth proposed solution is to provide a luminescence transistor that operates based on function as a junction transistor, as opposed to a TFT, which is a field effect transistor. By integrating junction transistor functions and organic EL functions, load on the drive transistor is reduced compared with using a TFT drive transistor.
A luminescence transistor based on a junction transistor function may be more realistic than a TFT function because unlike a TFT, a junction transistor does not confine current to a narrow channel. Consequently, junction transistor technology can create a pixel having a greater luminescence area than in the case of TFT. Such technology is disclosed in JP2005-293980, JP2006-269323, JP2007-200788, JP2009-272442, and JP2010-135809. Disclosed amongst these documents is a current control base electrode disposed in a middle portion of an organic semiconductor layer, in which the base electrode is a slit-like conductor, a planar conductor, or a conductor/semiconductor having a laminated structure. The disadvantage of these technologies is that a semiconductor layer having a current control base electrode disposed in the middle portion is difficult to manufacture.
This technology enables the use processes at lower temperatures (eg at or below 300° C.) than in some other technologies, for example in technologies requiring annealing. Such technology therefore has an advantage of allowing a separation between the luminescence transistor manufacturing process and the backplane manufacturing process.
However, the structure of having a conductor inserted into an organic semiconductor performs poorly as a junction transistor. Furthermore, the structure is unstable, and its instability becomes an impediment to volume production and to production of large panels.
It would be desirable to solve or at least ameliorate at least one of these problems of the prior art. Various embodiments of the invention may therefore provide a stable, inexpensive, and/or high-performance luminescence transistor or active matrix OLED display. Various embodiments may allow a separation between the luminescence transistor manufacturing process and the backplane manufacturing process, and/or reduce a current control load at the backplane.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.